The common collector connection of two transistor stages--the first of which sees as load the input conductance of the second, called Darlington's connection (Darlington pair)--is widely used in applications where it is necessary to obtain a very small value for the input admittance of an amplifier utilizing semiconductor devices of the bipolar kind.
As it is well known, the current gain stages of a Darlington's pair present a drawback which may exclude their utilization in many applications characterized by peculiar requirements; that is, the voltage between the collector terminal and the emitter terminal of the "equivalent transistor" in the Darlington's arrangement cannot fall below the value of the sum of the V.sub.BE of the second transistor (T.sub.2 in the figures) and of the V.sub.CE of the first transistor (T.sub.1 in the figures) in the cascade pair.
In the very frequent case where the output current of the Darlington is utilized, through a current mirror, for driving another circuit of any kind, it is often of fundamental importance that the voltage drop between the terminals C (collector) and E (emitter) of the Darlington be very small in order not to limit excessively the voltage swing, in other words, the dynamic characteristic of the E node or the C node. In fact, the current mirror may have a type of circuit implementation which may be complex; and this may bring forth a certain voltage drop which, summed to the voltage drop of the Darlington (that is between said C and E nodes), may be incompatible with the limit imposed by the supply voltage, which is often of only 5 V.
A similar inconvenient and intolerable excessive limitation of the excursion possibility of the voltage is encountered in output stages which utilize the Darlington's connection and a sensing resistor for detection and control, by a feedback, of the current through the output transistor. Also in this type of application the voltage drop between the C and E nodes of the Darlington limits the maximum voltage developable across the load of the circuit.
With the aim of overcoming such a drawback of Darlington's stages, particularly in circuit situations of the above mentioned type, and thus to reduce the voltage drop of the current gain stage, it is known to connect the collector of the first transistor T.sub.1 (driver), not to the collector of the second transistor T.sub.2, in accordance with the typical Darlington's connection, but to a circuit block, distinct from the "load" or from the current mirror driven by the output transistor T.sub.2, or even directly to the supply line.
This arrangement reduces the minimum voltage between said C and E nodes of the current gain stage only to the V.sub.CE voltage of the output transistor T.sub.2. But it also leads to the deterioration, though, of the precision of the output current, as it will be better illustrated later in the description. The degree of imprecision which is introduced through said known arrangement is intolerable in many applications.